1. Field of The Invention
This invention relates to analog-to-digital converters and, more particularly, to an analog-to-digital converter with an extended spurious free dynamic range (SFDR).
2. Description of Related Art
Many physical devices generate output signals which are analog or continuously varying. Today, signal processing is often accomplished using digital methods. In many applications it is required to convert an analog signal into a digital form suitable for processing by a digital system. Many types of converters exist which act as interfaces between analog devices and digital systems. These converters are used in a variety of applications, including testing, measurement, process control, and communications. Analog-to-Digital (A/D) converters produce a digital output from an analog input. In converting analog signals to digital form, the analog signal is typically sampled then quantized. Nonlinearities in the sampling and quantizing of the analog input signal by the A/D cause spurious distortion, such as intermodulation distortion, of the digitized signal. The amplitude of spurious distortion and the amplitude of the digital output signal can be measured by converting the digital output signal into the frequency domain, for example by using a fast fourier transform.
The spurious free dynamic range (SFDR) of an A/D converter is often measured as the difference in decibels (dBs) between the amplitude or power level of the spurious distortion, such as intermodulation distortion, and the amplitude of the highest output signal. For example, for discussion purposes, a 12 bit A/D can have an SFDR in the 80 dB range defined by a high output level of 4 dBm and a corresponding distortion level of -76 dBm. The SFDR performance of a A/D varies as a function of the amplitude of the analog signal to be converted. Usually, the greater the amplitude of the analog signal, the smaller the SFDR because as the amplitude of the analog input signal increases, the amplitude of the digital output signal increases, but the amplitude of the intermodulation distortion increases more than the amplitude of the digital output signal. Extending the SFDR of the A/D increases the relative difference between the amplitude of the digital output signal and the spurious distortion.
An A/D system with an extended dynamic range is desirable.